The present invention relates to an intense soft X-ray source using cylindrical plasma compression, the plasma being obtained from an exploded sheet.
The plasmas to which the invention relates are dense, hot plasmas. Their electron density exceeds approximately 10.sup.18 cm.sup.-3 and their electron temperature is in the range between a few hundred electron volts to a few kiloelectron volts.
Such plasmas can constitute intense soft X-radiation sources which, compared with other X-ray sources have numerous advantages, such as:
a) their low cost;
b) their overall dimensions are sufficiently reduced to enable them to be positioned at the point of use of the X-radiation;
c) easy to use and maintain;
d) high energy efficiency.
These advantages make such sources suitable for microlithography, whilst also being usable in fast X-ray microscopy.
Some of these sources make use of a cylindrical plasma compression and they are sometimes called "liners".
This known procedure has already been applied to devices produced industrially for microlithographic applications. Thus, the article entitled "X-ray lithography using a pulsed plasma source" published in the Journal of Vacuum Science Technology, 19, 4, November/December 1981, pp.1190-1193 by J. S. Pearlman and J. C. Riordan describes a soft X-ray source essentially comprising a means for producing a cylindrical supersonic plasma jet through a hollow cathode in the direction of an anode. A discharge circuit connects the cathode to the anode through a capacitor bank previously charged by a high voltage source. During the discharge of these capacitors through the plasma jet, there is a cylindrical compression of the latter and a soft X-ray emission results therefrom.
Such a source is also described in the article entitled "Intense plasma source for X-ray microscopy" published in the Journal "SPIE" "Society of Photooptical Instrumentation Engineers", Vol. 316, High Resolution Soft X-ray Optics, 1981, pp. 196 to 202 by R. A. Gutcheck and J. J. Muray. This article also describes a source using a ring of conductor wires whose explosion it causes the cylindrical compression of this ring then taking place.
The temperature and density of the plasmas obtained in such sources are essentially limited by the two following physical phenomena:
a magnetohydrodynamic instability developing in the compressed plasma and which leads to the use of very fast high voltage generators, so as not to allow this instability to develop, the essential parameter defining the performances of the source then being the initial homogeneity of the plasma to be compressed;
the compression is limited by the presence of gas within the cylinder to be compressed, which reduces the final temperature and density obtained.
These two limitations are important in the aforementioned known devices. The plasmas produced by a supersonic gas jet have a relatively good homogeneity, but interactions between the supersonic jet and the walls, electrodes, etc leads to shock waves in the jet, which introduce gas into the cylinder to be compressed. The plasmas produced by explosion of the wires have a mediocre homogeneity and also are unsuitable for machines with a modest power level.